1. Field of the Invention
This invention relates to a bioceramic coated apparatus and a method for forming the same on a nanoporous valve metal substrate. More particularly, the invention is directed to a method of forming a nanoporous valve metal substrate having a bioceramic coating formed thereon for use as a medical implant.
2. Background of the Invention
Calcium phosphate coatings have received much attention as the coatings have excellent biocompatibility properties and are widely used on medical implants including dental and orthopedic implants. For example, the coatings have excellent osseointegration properties. Several coating methods such as plasma spray, sol-gel, electrophoretic, RF sputtering and electrochemical deposition are being used to coat implant surfaces as described in M. C. Kuo, S. K. Yen, The Process of Electrochemical Deposited Hydroxyapatite Coatings On Biomedical Titanium at Room Temperature, Materials Science and Engineering, C, 20, (2002), pp. 153-160, which is incorporated by reference.
Plasma spray processes are currently widely used commercial methods for coating bioceramics on implants. However, coatings formed by plasma spray processes have several disadvantages, such as, micro-cracks, poor adhesion between the coating and substrate, phase changes due to high temperature exposure, non-uniformity in the coating density and improper microstructural control. These disadvantages may result in failure of the implanted system. See Rodriguez et al., In Vitro Osteoblast Response to Anodized Titanium and Anodized Titanium Followed by Hydrothermal Treatment, Biomed. Mater. Res., 37, (2003), pp. 352-358, which is incorporated by reference. Attempts have been made to improve the adhesion between the coating and the substrate. For example, in order to improve the adhesion between the bioceramic coatings and substrates, hydroxyapatite formation has been carried out in a series of hydrothermal treatments of anodized surface in a solution containing Ca and P. See Zhu, et al., Characterization of Hydrothermally treated Anodic Oxides Containing Ca and P on Titanium, J. Mat. Science: Materials in Medicine, 14, (2003), pp. 629-634, which is incorporated by reference. Anodic plasma chemical processes are another method to obtain and improve adhesion of anodized surfaces containing Ca and P. See Frauchiger, et al., Anodic Plasma-Chemical Treatment of CP Titaniun Surfaces for Biomedical Applications, Biomaterials, 25, (2004), pp. 593-606, which is incorporated by reference. However, in both the hydrothermally treated anodized surface process and the anodic plasma chemical process, there is discrete formation of apatite-like phases and the coating obtained is not uniform among other disadvantages. Also, in conventional anodic plasma chemical processes, the pores are formed during anodization at high voltages (e.g., >200 V) in calcium phosphate containing solutions. The pores were larger (1-5 microns diameter) and size of the pores was not uniform. Moreover, the large voltages leads during anodization resulted in dielectric breakdown of the oxide layer and internal microcracks. Subsequently, deposited hydroxyapatite crystals grown out of these discrete pores were not uniformly and densely distributed for better biocompatibility. Further, inherent defects of oxide film formed at higher anodization voltages over which hydroxyapatite crystals were formed lead to poor interfacial bond strength and poor bone integration.
A need therefore exists for improved bioceramic coated apparatuses and methods for making them. The apparatuses should have improved adhesion of the bioceramic coating.